Fluorescent dye loaded particles have been widely used in diagnosis, bioimaging and bioassay purposes. Various methods have been adopted in preparing the fluorescent particles, and particularly, layered double hydroxide (LDH) based particles have been attracting growing attention in this area. LDH is a layered material composed of bivalent and trivalent metallic cations (M2+ and M3+) in layers and negatively charged anions occupying the gallery in between. The LDH family can be expressed as [M2+1−xM3+x(OH)2][An−]x/n.zH2O, where M2+ may be Mg2+, Zn2+, Ni2+ or other bivalent cations and M3+ may be Al3+, Ga3+, Fe3+, Mn3+ or other trivalent cations; An− as a charge compensating anion may be CO32−, Cl−, etc. Since An− functions as non-framework anions, other types of negatively charged ions can easily enter the gallery between the LDH layers by ion-exchange.
Owing to the powerful anion exchange capacity, positive surface charge and excellent biocompatibility, LDH nanoparticles have been widely used as carriers of negatively charged genes such as DNA and siRNA for intracellular delivery and other biomedical applications. Negatively charged fluorescent dyes can be directly loaded into the LDH layers by anion exchange. However, by undergoing anion exchange, dye molecules uncontrollably fill in the gallery or attach to the surface of the LDH nanoparticles with very high local concentration, which leads to close spacing between molecules and a corresponding concentration quenching. As a result, the resultant nano-materials retain very low fluorescence efficiency in solution and even no fluorescence in dry form, which is undesirable.